The invention relates to a method and apparatus for trimming and stabilizing of an aircraft such as a helicopter. The control input signals for trimming and stabilizing an aircraft are proportional to the rotational speeds of the aircraft about its respective axis, such as the pitch axis, the rolling axis, or the yawing axis. The steering mechanism of the aircraft is conventionally equipped with a device for producing for the pilot a simulated sensation of the steering control force, whereby the zero position or neutral position of the steering control force corresponds to a stable attitude (no rate) of the aircraft relative to any of the mentioned axes. Potentiometer means may be used for generating a signal representing the neutral force position of the steering means.
Mechanisms, as described above, are known in the art. Thus, for example, helicopters equipped with a hydraulic power amplification in the steering mechanism also normally comprise a trimming system for the trimming and stabilization about the rolling and pitching axis. Such systems impart to the pilot through biased springs a simulated or artificial sensation or feeling for the steering force. The zero or neutral position of such steering control force is adjustable either by means of a releasable magnetic brake or by an electrical trimming motor, which is controlled through a displacement responsive switch.
German Patent Publicaton (DE-OS) No. 2,634,890 discloses a system for the adjustment of the position of a control surface in an aircraft. Starting at page 12 and the following pages, this reference describes a closed loop trimming control for automatically maintaining the instantaneous position of a control lever. This type of system is said to be compatible with the increase of the control sensation and the reduction of the load on the rotors of a helicopter. However, this type of closed loop trimming control does not take into account the instantaneous position of an aircraft or rather the instantaneous rotational speed of the aircraft about its movement relative to any of the above mentioned axes. Thus, in the device of the prior art the trimming takes place only semi-automatically in response to the operation of a control member, such as a control button. Besides, the known system is not intended to maintain any new instantaneous attitude of an aircraft.
U.S. Pat. No. 4,198,017 (Murray) discloses a control augmentation system for flight vehicles in which angular velocities are controlled in proportion to a controlled input. A stabilization of the corresponding vehicle attitude is also achieved by Murray. By using a "feed forward" signal the controllability factors available to the pilot are augmented. The feedback path augments the vehicle's inherent stability. As shown in FIG. 2 of Murray the control stick is connected to a parallel trim actuator through a spring. However, Murray does not teach the derivation of a q-model signal from the spring connecting the control stick to the parallel trim actuator.
U.S. Pat. No. 4,281,811 (Nixon) discloses a pitch trim system for an aircraft in which the pitch trim system responds to an output of a torque detector providing an input signal to a gain normalization circuit. Providing a simultaneous trimming of the control surface of an aircraft and applying a simulated sensation of the steering control forces to the pilot holding the control stick is not disclosed by Nixon.
U.S. Pat. No. 4,500,967 (Murphy et al) discloses an aircraft short term roll attitude retention system in which a roll attitude retention outer loop actuator, a roll stability inner loop actuator, and a control stick are used for positioning control surfaces of the aircraft to control the roll attitude. A spring is connected between the control stick and a roll retention hydraulic servo. A first position sensor is connected to one end of the spring and a second position sensor is connected to the other end of the spring. A feed back signal is produced including the signal provided by the first sensor for the roll retention hydraulic servo. Additionally, both sensors are connected to a summing circuit which produces a signal for switching an integrator, and to provide an indication that the pilot is controlling the roll attitude of the aircraft against the trim position established by the servo actuator. Such a system also does not provide the simultaneous simulated sensation of the steering control force to the pilot together with the automatic trimming.